4. Consider the following chemical reaction: a) Calculate the equilibrium consta

Question

4. Consider the following chemical reaction: a) Calculate the equilibrium constant at 1 atm and 298 K for the formation of gaseous hydrofluoric acid. Use the standard enthalpy and entropy changes for this reaction to do this calculation. The relevant tables are in McQuarrie Ch. 19 and 21. b) Now calculate the equilibrium constant at 1000 K, assuming the standard reaction enthalpy does not depend on the temperature. c) Suppose that the composition of the mixture in terms of the partial pressures of the gases at some point in time is found as follows:

Explanation / Answer

Enthalpy of reaction = sum of heat of formation of products - sum of heat of formation of reactants

= 0.5* (-273.3) - (0 + 0)

H = - 136.65 kJ/mol

Entropy of reaction = sum of entropy of formation of products - sum of entropy of formation of reactants

= 0.5* (173.8) - (130.7 + 202.8)

S = - 246.6 J/mol-K = - 0.2466 kJ/mol-K

Gibbs energy change of reaction at 298 K

G = H - TS

= - 136.65 + 298*0.2466

= 63.1632 kJ/mol = - 63163.2 J/mol

Equilibrium constant at 298 K

G = - RT ln K

K = exp (-G/RT)

= exp (63.1632*1000/8.314*298)

= exp(25.49)

K = 1.179 x 10^11

Part b

Gibbs energy change of reaction at 1000 K

G = H - TS

= - 136.65 + 1000*0.2466

= 109.95 kJ/mol = 109950 J/mol

Equilibrium constant at 1000 K

G = - RT ln K

K = exp (-G/RT)

= exp (-109950/8.314*1000)

= exp(-13.224)

K = 1.806 x 10^-6

Part C

Reaction quotient

Q = (PHF)0.5 / (PH2 x PF2)

= (4)0.5 / (0.5*1)

= 4

Q > K

Products to reactants ratio is high

System must shift towards reactants to reach equilibrium

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